#include <bits/stdc++.h>

using namespace std;

struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};

class Solution
{
public:
    // vector<vector<int>> levelOrder(TreeNode *root)
    // {
    //     vector<vector<int>> res;
    //     queue<TreeNode *> que;
    //     if (root != nullptr)
    //     {
    //         que.push(root);
    //     }
    //     while (!que.empty())
    //     {
    //         int length = que.size();
    //         vector<int> vec;
    //         for (int i = 0; i < length; i++)
    //         {
    //             TreeNode *node = que.front();
    //             que.pop();
    //             vec.push_back(node->val);
    //             if (node->left)
    //             {
    //                 que.push(node->left);
    //             }
    //             if (node->right)
    //             {
    //                 que.push(node->right);
    //             }
    //         }
    //         res.push_back(vec);
    //     }
    //     return res;
    // }

    void order(TreeNode *node, vector<vector<int>> &vec, int deep)
    {
        if (node == nullptr)
        {
            return;
        }
        if (vec.size() == (size_t)deep)
        {
            vec.push_back(vector<int>());
        }
        vec[deep].push_back(node->val);
        order(node->left, vec, deep + 1);
        order(node->right, vec, deep + 1);
    }

    vector<vector<int>> levelOrder(TreeNode *root)
    {
        vector<vector<int>> res;
        int deep = 0;
        order(root, res, deep);
        return res;
    }
};

int main()
{
    TreeNode *root = new TreeNode(3);
    TreeNode *node1 = new TreeNode(9);
    TreeNode *node2 = new TreeNode(20);
    TreeNode *node3 = new TreeNode(15);
    TreeNode *node4 = new TreeNode(7);

    root->left = node1;
    root->right = node2;
    node2->left = node3;
    node2->right = node4;

    Solution sol;
    vector<vector<int>> res = sol.levelOrder(root);
    for (auto vec : res)
    {
        for (auto num : vec)
        {
            cout << num << " ";
        }
        cout << endl;
    }
    return 0;
}